1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer and a facsimile machine, having a development device that is configured to develop an electrostatic image formed on an image bearing member using electrophotography, an electrostatic recording method, or the like.
2. Description of the Related Art
In image forming apparatuses that use electrophotography such as a copying machine, an electrostatic latent image formed on an image bearing member such as a photosensitive drum is visualized by depositing developer on the image. In the conventional development devices used for the development, development devices that use two-component developer have been known. The two-component developer is composed of toner and carrier. FIG. 10 is a cross sectional view illustrating an example of a conventional development device.
As illustrated in FIG. 10, a development device 101d that uses the two-component developer generally has two carrying screws arranged in a horizontal direction, that is, a first carrying screw 105 and a second carrying screw 106 that carry the two-component developer while agitating the developer. The first carrying screw 105 supplies the developer to a developer bearing member (hereinafter, referred to as development sleeve) 108 and collects the developer after the developer passes through a development area. The second carrying screw 106 mixes and agitates the developer collected by the development sleeve 108 and newly supplied developer.
Meanwhile, in recent years, in order to achieve space efficiency, a demand for downsizing the apparatus bodies of the image forming apparatuses is increasing. Especially, since full color image forming apparatuses have several development devices, the downsizing of the bodies is highly demanded. For this purpose, as discussed in Japanese Patent Application Laid-Open No. 5-333691, a vertical agitation type development device, for example, have been proposed, as illustrated in FIG. 11.
A development device 111 shown in FIG. 11 has a structure different from the development device 101d illustrated in FIG. 10, that is, carrying screws that function as carrying members for agitating and mixing two-component developer are vertically arranged in the direction of gravitational force. A first carrying screw 115 conveys the developer in a development chamber 113 that is arranged at an upper side of a development container and supplies the developer to a development sleeve 118. A second carrying screw 116 mixes and agitates the developer collected from the development sleeve 118 and newly supplied developer while carrying the developer in an agitation chamber 114 that is arranged at a lower side of the development container.
As described above, in the vertical agitation type development device, the development chamber and the agitation chamber are vertically arranged, and the occupancy space in the horizontal direction is small. Accordingly, for example, it is possible to downsize tandem type color image formation apparatuses that have a plurality of development devices arranged in parallel in the horizontal direction.
In the above development method that uses the two-component developer, an image is formed by applying electric charge to toner with frictional electrification of carrier and toner, and electrostatically attaching the charged toner to an electrostatic image. In the two-component development method, in order to provide highly durable and stable images, it is desirable to stably apply a certain amount of toner charge, that is, triboelectrification (hereinafter, simply referred to as tribo). For that purpose, it is desirable that the charging ability of the carrier is maintained to be stable between before and after prolonged use.
However, in reality, while the toner is constantly consumed in the development, the carrier remains in the development device without being consumed. Accordingly, in long-term use, the carrier is agitated with the toner for a long time, and the surface of the carrier can be contaminated by the toner or an external additive of the toner. As a result, after prolonged use, the ability of the carrier for supplying the tribo may be decreased and the tribo may be degraded. This can cause image deterioration such as fog and toner scatter.
Therefore, for example, as discussed in Japanese Examined Patent Application Laid-Open No. 2-21591, a device that can control the deterioration of the charging ability by supplying developer to development devices is proposed. That is, the supply device supplies new developer or carrier, and excessive developer in the development device is discharged from a developer discharging opening disposed on a wall surface of the development device. Then, the discharged developer is collected.
The devices sequentially repeat the supply of the new developer or carrier as well as the discharge of the developer to replace the deteriorated developer in the development devices with the newly supplied toner and carrier. Accordingly, the development characteristic of the developer in the development devices is generally maintained at a constant characteristic, and the image quality deterioration can be reduced. As a result, intervals of the replacement of the developer by serving staffs can be extended or the replacement can be eliminated.
However, if the developer discharge method discussed in Japanese Examined Patent Application Laid-Open No. 2-21591 is applied to an image forming apparatus having the above-described vertical-agitation development device, the following problems may occur.
In the case where developer is discharged in the vertical-agitation development device, a discharge opening of the developer can be provided on a wall surface of a container in a development chamber at a downstream side in a developer conveyance direction, and from the discharge opening, overflow developer is discharged and collected. If the discharge opening is provided at a place other than the wall surface, for example, in an agitation chamber, new carrier that has just been supplied may be immediately discharged. Alternatively, the discharge opening can be provided in the development chamber at an upstream side in the developer conveyance direction. However, in the above-described configuration, the position of the discharge opening is right behind a position where the developer is lifted from the agitation chamber to the development chamber, and a height of the developer surface in the development chamber may not be stable. Accordingly, appropriate discharge of the developer may not be performed.
As described above, in the case where the discharge opening of the developer is provided in the agitation chamber at the downstream side in the developer conveyance direction, if the developer surface exceeds the height of the discharge opening, the developer may overflow and be discharged. However, if the developer surface does not exceed the height of the discharge opening, the developer may not be discharged.
Meanwhile, in a case of forming an image on heavy paper or overhead transparency (OHT), some image forming apparatuses slow down the image formation speed as compared with normal image formation. For example, some image forming apparatuses have a deceleration mode that decreases an image formation speed to, for example, a half or one-third of a normal speed. In the deceleration mode in which a rotation speed of a photosensitive drum is decreased, generally, a drive speed of a development device is similarly decreased to, for example, a half or one-third of the normal drive speed.
In a case where a drive speed is decreased in the vertical-agitation development device in the deceleration mode, the number of rotations of the first and second carrying screws are reduced as compared with a normal mode. Accordingly, a developer conveyance speed is decreased. As a result, a development conveyance speed of the second carrying screw is also decreased, so that momentum of the developer to ascend from the agitation chamber to the development chamber is also decreased, and the amount of the developer lifted to the development chamber is decreased, too. Accordingly, in response to the decrease of the developer amount in the development chamber, the developer surface descends lower than in the normal case. Accordingly, the developer is not easily discharged from the developer discharge opening provided at the downstream side of the development chamber. Consequently, if the deceleration mode is continued, the amount of the developer in the development device is gradually increased. Accordingly, the developer accumulates especially at the lift portion in the agitation chamber.
If such a state is maintained, the pressure of the developer is increased at the lift portion, and the developer can leak from a lower part of the development sleeve or torque can be increased, which may cause the carrying screw to lock.
To avoid the problems, the developer discharge opening may be provided at a relatively lower part in consideration of the height of the developer in the development chamber at the time of the deceleration mode. However, in such a configuration, the developer may be excessively discharged in the normal image formation mode and the amount of the developer in the development device may become too small. Then, toner density may show a great variation when the toner is supplied, or the developer may be easily deteriorated.